1. Field of the Invention
The invention relates to an apparatus having multiple heating wires for heating of a flat workpiece made of thermoplastic material within a region of a subsequent deformation of the workpiece.
2. Description of the Prior Art
Workpieces made of thermoplastic material must be plastified prior to deformation, for example prior to bending, by heating. Such heating should be limited locally and be accomplished in a contactless manner, particularly in the case of glasslike transparent synthetic plastic materials.
A heating apparatus for local heating of plates made of thermoplastic synthetic is known, wherein the heat source consists of two juxtaposed heating swords or blades, whose cutting edges abut a plate spaced and gripped therebetween. This apparatus is, however, unsuitable, for the bending of glasslike transparent materials, since the cutting edges of the blades are pressed into the material deforming the same, the deformations remaining annoyingly visible on the completed workpiece.
This deficiency no longer occurs in another known heating apparatus, which operates in a contactless manner by means of heating rods or bars. But it is not possible even by means of this apparatus to heat glasslike plates of synthetic plastic material in such a fashion that the bent edge is devoid of any deficiencies. Experiments have proved that the heating rods of a great many different manufacturers have shown considerable temperature variations over their length. Of 100 heating rods examined only 3 showed a somewhat uniform temperature distribution, and only to the extent that they were new. A workpiece made of acrylic glass of 10 mm thickness preheated with such heating rods and subsequently bent showed considerable refraction phenomena upon a light passing therethrough in the form of nonuniform bending radii, whereby it became unsuitable for its intended application.